Retroreflective materials are employed for various safety purposes such as highway signs, street signs, pavement markings, tape and patches on clothing which are highly visible to a driver of a car at night. Particularly, these materials are useful at night time when visibility is critical under low light conditions.
One type of retroreflective material is formed of cube-corner or prism retroreflectors, such reflectors, are described in U.S. Pat. No. 3,712,706, issued to Stamm (Jan. 23, 1973). Generally, the prisms are made by forming a master die on a flat surface of a metal plate. To form the cube corners, three series of parallel equidistance intersecting v-shaped grooves at 60.degree. angles to each other are inscribed in the flat plate. An electroformed opposite shape of the die is then used to form the desired cube-corner array into a flat plastic surface. When the groove angle is 70 degrees, 31 minutes, 43.6 seconds, the angle formed by the intersection of two cube faces (the dihedral angle) is 90 degrees, and the incident light is retroreflected back to the source.
The efficiency of a retroreflective structure is a measure of the amount of incident light returned within a cone diverging from the axis of retroreflection. Distortion of the prism structure adversely affects the efficiency. When faces of the cube corners are air backed (not metalized to achieve specular reflection) and where corners are not tilted in an array to achieve improved angularity, cube-corner retroreflective elements have low angularity, i.e., the elements will only brightly reflect light that impinges on it within a narrow angular range centering approximately on its axis of retroreflection. Low angularity arises from the inherent nature of these elements which are trihedral structures having three mutually perpendicular lateral faces. Cube-corner elements operate according to the principle of total internal reflection when air backed and to the principle of specular reflection when backed with a reflective metal coating. A cube-corner element receives a ray of incident light from a source and sends it back toward the same source in a direction that is substantially parallel to the ray of incident light. The elements are arranged so that the light to be retroreflected impinges into the internal space defined by the faces, and retroreflection of the impinging light occurs by total internal reflection of the light from face to face of the element. Impinging light that is inclined substantially away from the axis of retroreflection of the element (which is the trisector of the internal space defined by the faces of the element) strikes the face at an angle less than its critical angle, thereby passing through the face rather than being reflected.
Further details concerning the structures and the operation of cube-corner microprisms can be found in U.S. Pat. No. 3,684,348, issued to Rowland (Aug. 15, 1972), incorporated in its entirety by reference herein. A method for making retroreflective sheeting is also disclosed in U.S. Pat. No. 3,689,346, issued to Rowland (Sep. 5, 1972), and the casting of the ultraviolet cured retroreflective microprisms on a substrate or mold is described in U.S. Pat. No. 3,810,804, issued to Rowland (May 14, 1974). The teachings of both references are incorporated by reference herein. The method disclosed in U.S. Pat. No. 3,689,346, teaches forming cube-corner microprisms in a cooperatively configured mold. The prisms are bonded to sheeting to form a composite structure in which the cube-corner formations project from one surface of the sheeting. The molds are commonly nickel electroforms either formed by the "pin bundle" technique or are nickel electroforms of engravings made by precision ruling machines.
One method for forming retroreflective sheeting is by compression molding polymeric sheeting against the electroform. In a second method, sheeting can be formed by casting oligomers against the electroforms, laminating a substrate film over the oligomer, and radiation curing the oligomer. A disadvantage of nickel electroform molds is that nickel, which is soft and malleable, is very easily scratched and disfigured from handling. Further, forming the electroform molds is expensive and time consuming.
For continuous casting of retroreflective sheeting, the nickel electroforms are fabricated into a continuous belt. However, the fabrication and mounting steps offer many opportunities to scratch and dent the fragile surface of the nickel electroform.
Therefore, a need exists for a reusable mold for casting a retroreflective sheet having an array of prism elements that overcome the disadvantages of described above.